In U.S. patent application Ser. No. 670,887, the applicant has already proposed a power transmission apparatus which is applied to a vehicle of the kind having its front and rear wheels driven by the same engine. In the proposed power transmission apparatus, a rotary shaft connected with the front wheels is coupled by means utilizing fluid pressure such as oil hydraulic pressure to another rotary shaft connected with the rear wheels.
FIG. 13 is a schematic sectional view of part of the power transmission apparatus proposed in the U.S. Patent Application cited above. Referring to FIG. 13, a housing 20 is coupled to a first rotary shaft connected with front wheels of a vehicle, and a rotary 19 is coupled to a second rotary shaft 14 connected with rear wheels of the vehicle. The rotor 19 is housed in the housing 20. A plurality of movable vanes 18 (10 vanes in this case) protrude from the outer peripheral surface of the rotor 19 to make sliding contact with the inner peripheral surface of the housing 20. A plurality of, or, for example, three pump chambers 36, 37 and 38 are defined between the housing 20 and the rotor 19, and ports 22, 23, 24, 25, 26 and 27 open at both ends of the pump chambers 36, 37 and 38 respectively. The ports 22, 24 and 26 communicate with each other through a first hydraulic oil passage OL.sub.1, and the ports 23, 25 and 27 communicate with each other through a second hydraulic oil passage OL.sub.2. Check valves 28 and 29 permit flow of hydraulic oil from a hydraulic oil reservoir 30 to the first and second hydraulic oil passages OL.sub.1 and OL.sub.2 only respectively. An orifice 33 is provided in a hydraulic oil passage 34 through which the first and second hydraulic oil passages OL.sub.1 and OL.sub.2 communicate with each other.
When there occurs a rotation speed difference between the first rotary shaft and the second rotary shaft 14, that is, between the housing 20 and the rotor 19, and the rotor 19 rotates relative to the housing 20 in a direction as, for example, shown by the arrow in FIG. 13, the ports 22, 24 and 26 act as delivery ports, and the hydraulic oil is delivered by the vanes 18 into the first hydraulic oil passage OL.sub.1 from the pump chambers 36, 37 and 38. On the other hand, the ports 23, 25 and 27 act as suction ports, and the hydraulic oil is sucked into the pump chambers 36, 37 and 38 from the second hydraulic oil passage OL.sub.2. That is, the element described above operate as a pump. The hydraulic oil delivered into the first hydraulic oil passage OL.sub.1 flows into the second hydraulic oil passage OL.sub.2 through the hydraulic oil passage 34, and, during flow of the hydraulic oil through the hydraulic oil passage 34, the hydraulic oil flow is encountered with a resistance provided by the orifice 33, which resistance is dependent upon the flow rate of hydraulic oil. Therefore, when a rotation speed difference occurs between the housing 20 and the rotor 19 due to slip occurred between the front wheels and the rear wheels, the resistance corresponding to the flow rate of the hydraulic oil due to the rotation speed difference is added and acts to decrease the rotation speed difference thereby achieving the state of four-wheel drive. Any leakage of hydraulic oil from the sealed parts of the housing 20 and rotor 19 during the above operation is made up by hydraulic oil supplied from the hydraulic oil reservoir 30.
Further, hydraulic oil chambers 59 are provided at portions of the rotor 19 receiving the radially inner ends of the vanes 18 respectively. These hydraulic oil chambers 59 can communicate with the first and second hydraulic oil passages OL.sub.1 and OL.sub.2 through check valves 39 and 40 which permit flow of hydraulic oil toward the hydraulic oil chamber 59 only. Therefore, when the rotor 19 rotates relative to the housing 20 in the direction shown by the arrow, hydraulic oil delivered from the first hydraulic oil passage OL.sub.1 (the second hydraulic oil passage OL.sub.2 when the rotor 19 rotates in the opposite direction) is introduced through the check valve 39 (40) into the hydraulic oil chambers 59. Thus, the vanes 18 are forced radially outward and pressed onto the inner peripheral surface of the housing 20 to improve liquid-tightness during the pumping operation.
Hydraulic oil delivered from the first or second hydraulic oil passage OL.sub.1 or OL.sub.2 is introduced into the hydraulic oil chambers 59 in the prior art power transmission apparatus as described above, so as to press the vanes 18 onto the inner peripheral surface of the housing 20 thereby improving liquid-tightness in the pumping operation mode. However, this function has not necessarily been fully exhibited. That is, the arrangement is such that, after feeding all of delivered hydraulic oil into the first hydraulic oil passage OL.sub.1 or second hydraulic oil passage OL.sub.2 formed in the housing 20, the delivered hydraulic oil is supplied to the second hydraulic oil passage OL.sub.2 or first hydraulic oil passage OL.sub.1 acting as the suction oil passage and is also introduced into the vane-lifting hydraulic oil chambers 59. Thus, a large quantity of hydraulic operating fluid flows through the circuit including the first hydraulic oil passage OL.sub.1 and second hydraulic oil passage OL.sub.2. Accordingly, the hydraulic pressure tends to become lower due to the resistance of the oil passages, and the pressure of hydraulic oil supplied to the hydraulic oil chambers 59 becomes lower than that of hydraulic oil delivered from the pump chambers 36, 37 and 38. As a result, the required liquid-tightness may not necessarily be maintained between the vanes 18 and the inner peripheral surface of the housing 20. In such a case, an increase in the pressure of delivered hydraulic oil (the torque-transmission hydraulic oil pressure) beyond a certain value causes radially inward movement of the vanes 18 away from the inner peripheral surface of the housing 20, and leakage of hydraulic oil from the delivery side chambers toward the suction side chambers of the pump chambers gives rise to a variation of the torque being transmitted between the housing 20 and the rotor 19 as shown in FIG. 14.
Further, the pressure of hydraulic oil fed from the delivery ports toward the suction ports through the first hydraulic oil passage OL.sub.1 or second hydraulic oil passage OL.sub.2 tends to have a large negative value depending on its flow rate. In such a case, cavitation tends to occur in the neighborhood of the ports acting as the suction ports, resulting also in a variation of the torque being transmitted between the housing 20 and the rotor 19.